The present invention relates to a subscriber line interface circuit for use in a telecommunication system or exchange that supports a two-wire subscriber line.
A subscriber line interface circuit normally has a power supply comprising a DC voltage source having its negative terminal connected to the a-wire or ring lead of the subscriber line and a positive terminal connected to ground and to the b-wire or tip lead of the subscriber line. The power supply also includes a resistance/capacitance bridge in the form of a first resistor inserted in the a-wire (negative potential side) and a second resistor inserted in the b-wire (ground potential side) and a capacitor which bridges the two wires at the terminals of the two resistors on the sides opposite to the respective potential sources. This power supply is coupled on the capacitor side with a two-to-four wire hybrid transformer for the two wire/four wire transition to the system.
Subscriber line interface circuits of this kind--that is, those which contain a power supply individual to the subscriber--are provided in exchanges which contain an electronic switching matrix. In such telecommunication networks, it is necessary to guard against an overload of the circuit elements contained in the power supply bridge which may result, for example, from faulty insulation of the two-wire subscriber line.
Overdimensioning of the power supply bridge, a conceivable solution to the problem of making the bridge less susceptible to overloads, would substantially reduce the number of subscriber line interface circuits that could be combined in one subassembly.
It is therefore conventional to provide a subscriber line interface circuit with an overload protection circuit. Normally, such a protection circuit senses the voltage drop across a small resistance in series with one wire of the subscriber line and, when this voltage exceeds some predetermined value, causes the wire to be disconnected from the supply. Once the circuit is interrupted, the voltage across the sensing resistor disappears, so the protection circuit effectively forms a latch. After the overload has been removed, the protection circuit must be reset.